1. Field of the Invention
The invention relates to a flat panel display, and more particularly, to a liquid crystal display (LCD) and a shift register device thereof.
2. Description of Related Art
In recent years, with great advance in the semiconductor technique, portable electronic devices and flat panel displays (FDPs) have been rapidly developed. Among various types of FDP, liquid crystal displays (LCDs) have gradually become the mainstream display products due to the advantages such as a low operating voltage, free of harmful radiation, light weight and small and compact size. As a consequence, a fabricating method is developed towards miniature and low cost by manufacturers in this field. In order to lower down the fabricating cost of LCDs, instead of disposing shift registers of a scan driver IC on a scan side of an LCD, parts of manufacturers can directly dispose the shift registers of the scan driver IC on a glass substrate of the LCD under an amorphous silicon (a-Si) process. Hence, the scan driver IC originally disposed on the scan side of the LCD can be omitted such that the fabricating cost of the LCD is reduced.
In general, each of the shift registers directly disposed on the glass substrate of the LCD includes a plurality of N-type transistors. Parts of the N-type transistors are responsible to output scan signal during operation period of the shift register so as to turn on the corresponding pixel row in the LCD panel. And other N-type transistors are responsible to stop outputting the scan signal during non-operation period of shift registers.
However, in practice, the channel lengths of the N-type transistors responsible to output the scan signal and the channel lengths of the N-type transistors responsible to stop outputting the scan signal are the same. Hence, under this condition, the N-type transistors responsible to output the scan signal may not output the scan signal normally due to the leakage currents of the N-type transistors responsible to stop outputting the scan signal when the N-type transistors being responsible to stop outputting the scan signal are in the sub-threshold region. As a result, the shift register malfunctions and the LCD is not able to display a display image normally.